Coagulation Measurement and Optimisation in Cardiac Surgery

Coagulation is a complex physiological balancing mechanism that maintains haemostasis when blood vessels are injured. A hypercoaguable state as seen in early sepsis, cancer or pregnancy can cause arterial and/or venous obstruction with disruption of blood-flow and end organ damage. Similarly a hypocoaguable state can cause catastrophic haemorrhage and lead to death. Up to 20% of all blood transfusions in the USA are related to cardiac surgery (1). The pathogenesis of coagulopathy in this patient group is multifactorial: patients are older (2), and more complex and this is reflected in the number of redo surgeries and the concomitant use of agents such as clopidogrel, aspirin, coumarin anticoagulants and heparins. Also cardiopulmonary bypass activates the coagulation system with an initial hypercoaguable state and platelet activation, followed by factor and platelet consumption. Hypothermia, acidosis, hypocalcaemia and the dilutional effects of circuit priming all increase the risk of bleeding (3). Platelet transfusion per se in the perioperative period has been associated with an increased risk of serious adverse events (4). Indeed both red cell and platelet transfusion have been shown to have a negative risk-adjusted effect on health-related quality of life after cardiac surgery (5). The haemostatic status of a patient undergoing cardiopulmonary bypass can change very quickly because of haemorrhage or the use of high dose heparin or protamine and as such blood component administration in cardiac surgery can often be empiric. This is compounded by the limited utility of the standard coagulation tests, which have a slow turnaround time in a setting where there can be rapid changes in coagulation status. All patients presenting for cardiac surgery will be anticoagulated in the perioperative period, either for cardiac or non-cardiac reasons, and an appropriate balance needs to be struck between minimising perioperative blood loss and use of homologous blood products, and avoiding pathological thrombosis. Coagulation status can be measured by means of laboratory tests or near-patient tests (also referred to as point-of-care tests). Effective use and interpretation of these tests can guide physicians and surgeons alike in the use of medications and homologous blood products and timely intervention of surgery to optimise patient outcomes. Indeed, the thromboelastogram (TEG), a point-of-care test, should discriminate “surgical” from “medical” causes of bleeding in this population thus reducing unnecessary transfusions and allowing timely return to the operating room for definitive treatment. The TEG will also

are useful in terms of interpreting some of the coagulation tests, but in vitro coagulation is less linear and occurs by a combination of these pathways. Current understanding of the coagulation process is best described by the cell-based model. The coagulation factors were discovered in the 1940s and 1950s. They are proenzymes found in plasma, which are converted to active enzymes during the coagulation process. The factors were assigned roman numerals in the order they were discovered; each factor also has one or more names. The more commonly used ones are shown in the figure 2 (6). The numerals are prefixed by "F" for "factor", and the suffix "a" is used to indicate the activated form of the factor.

Intrinsic pathway
The "Intrinsic Pathway", so-called because its components are all present within whole blood, is initiated when blood comes in contact with a negatively charged surface such as glass or the surface of an activated platelet. It consists of a reaction cascade involving FXII, High Molecular Weight Kininogen (HMWK), prekallikrien, FXI, FIX, FIIa, FVIII, and Ca 2+ and culminates in the activation of FX at the Common Pathway. The activated partial thromboplastin time (APTT) laboratory test measures the intrinsic pathway.

Extrinsic pathway
The "Extrinsic Pathway" is initiated when endothelial injury causes tissue factor (TF, or FIII) to come into contact with FVII. TF non-proteolytically activates FVII. The FVIIa/TF complex then activates FX. The extrinsic pathway can be measured by the Prothrombin Time (PT) laboratory test.

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Perioperative Considerations in Cardiac Surgery 182

The common pathway
The intrinsic and extrinsic pathways both lead to activation of FX. FXa, FVa and Ca 2+ activate prothrombin (FII) to form thrombin (FIIa). Thrombin has several actions: it activates fibrinogen (FI) to yield fibrin monomers; it amplifies the coagulation process by activating further prothrombin, FV and FVIII; it activates FXIII whose active form promotes cross-linking of fibrin monomers to form stable polymers; (as discussed later in the cell-based model, it also participates in processes that temper coagulation to avoid overwhelming pathological thrombosis.) The common pathway thus yields a mesh of covalently cross-linked fibrin strands and entrapped red blood cells, which form a stable blood clot. Disorders of the common pathway will affect tests of the intrinsic and extrinsic pathways.

The cell-based model
The five steps of this model are initiation, amplification, propagation, stabilisation and inhibition of further coagulation (7). In health, 1% of circulating FVII is in the active form.
Blue, Red and Purple correspond respectively to Intrinsic, Extrinsic and Common Pathways of the Classic Coagulaton Cascade. Thrombin has a key role throughout, hence is shown in yellow. TF = Tissue Factor. vWF = von Willebrand Factor. TAFI = Thrombin Activated Thrombolysis Inhibitor. TFPI = Tissue Factor Pathway Inhibitor. At 'initiation', TF binds to circulating FVIIa and acts with FV to generate FIXa and FXa. The FXa converts a small amount of FII to thrombin.  During 'amplification' this thrombin triggers reactions on the surface of activated platelets, where more FVIIa is produced. Thrombin activates co-factors FV and FVIII. These active factors facilitate the sequence of thrombin-producing reactions.  In 'propagation', the TF/FVIIa complex continues to activate FIX and the FIXa/FVIIIa ("tenase") complex generates more FXa to drive a continuous "thrombin burst".  In 'stabilisation', FXIII is activated by high levels of thrombin and polymerises the fibrin monomers to complete the clot. Thrombin-activated Fibrinolysis Inhibitor contributes to clot stability.  Pathological thrombosis is prevented by regulation of the coagulation process. 'Inhibition of further coagulation' occurs via activated protein C (aPC), protein S, tissue factor pathway inhibitor (TFPI) and antithrombin. Thrombin activates protein C, which cleaves FVa and FVIIIa with Protein S acting as a cofactor. TFPI binds TF/FVIIa and FXa in a quaternary complex. Antithrombin inhibits thrombin, FIXa and FXa. (8,9)

Platelet function in haemostasis
Platelets respond to vascular wall damage by adhesion, aggregation, release of granule content and morphological changes. Von Willebrand Factor is a plasma protein that acts as a bridge between platelets and the exposed collagen of damaged tissue. The platelet binding site is the membrane glycoprotein GPIb-VIX. Von Willebrand Factor also acts as a carrier protein for, and stabiliser of FVIII, preventing premature proteolytic degradation by Protein C.

Hypercoagulability / Thrombophilia
A hypercoagulable state can predispose a patient to deep vein thrombosis (DVT), pulmonary embolism (PE), stroke, and recurrent miscarriage. Causes include disorders of platelets, vessel walls, systemic disease and well as genetic and environmental factors (10).

Haemophiliae and other Bleeding Diatheses
Disorders of coagulation factors tend to present with haemarthroses and muscle haematomas. Patients with platelet disorders more commonly present with skin bruising, epistaxis or menorrhagia. Von Willebrand's Disease is the most common inherited coagulopathy; other forms are rare, apart from Haemophilia A and B. The most common acquired coagulation disorders are disseminated intravascular coagulation (DIC), liver disease and vitamin K deficiency although disorders discussed elsewhere in this chapter are more common in the setting of cardiac surgery.

Anticoagulant drugs
These drugs are used to prevent thrombus formation, or limit the extension of existing thrombus. Venous thrombus consists of a fibrin web with trapped platelets and erythrocytes and is more successfully treated or prevented with anticoagulants than arterial thrombus, which contains proportionally more platelets and less fibrin.

Source
Hypercoagulable risk factor (mechanism and notes in brackets)

Thrombin inhibitors
Thrombin (FIIa) is central to the coagulation process. It usually converts soluble fibrinogen to fibrin, stimulates platelets and activates FV, FVIII, FXI and FXIII. Thrombin inhibitors can work directly on FIIa, or indirectly by preventing activation of FII.
Heparin is long-established as an anticoagulant, which facilitates endogenous antithrombin to decrease activation of FII. The indirect mechanism has limited activity against thrombin that is bound to fibrin or fibrin degradation products (FDPs). In contrast, direct-acting thrombin inhibitors (DTIs) inhibit bound, as well as free, FIIa. Heparin remains the most widely used drug in this class, but other agents are being investigated that may offer increased efficacy or a more favourable side-effect profile. Thrombin inhibitors may also have administrative advantages over heparin if they can be given orally and if anticoagulation monitoring is unnecessary.

Heparin
Heparin is found in the liver and mast cell granules. Commercial heparin is extracted from bovine or porcine sources and is a mixture of acid mucopolysaccharides with molecular weights ranging from 3000 to 60,000 daltons. It is available in this unfractionated form and also as various low-molecular weight preparations. Heparin reversibly binds to antithrombin III, thereby enhancing inhibition of FIX, FX, FXI, FXII, FXIII, thrombin and also plasmin. At high concentrations heparin also inhibits platelet aggregation. Unfractionated heparin is given intravenously or subcutaneously and is also used to anticoagulate the cardiopulmonary bypass circuit in cardiac surgery. Intravenous doses are titrated to a target range of ACT or APTT. The biological half-life is 1hr at physiological temperature but longer at lower temperatures during cardiopulmonary bypass. Heparin effects are reversed by protamine, a base that forms a stable, inactive salt complex with the acidic heparin. Low-molecular weight heparin preparations have an average molecular weight below 8000 daltons. They are more effective at inhibiting factor Xa than unfractionated heparin, with less of an effect on thrombin. Their longer half-life and more predictable pharmacokinetic profile make them suitable for once-daily subcutaneous dosing without routine coagulation monitoring. APTT is not altered by LMWH so if monitoring is needed, anti-Xa levels are measured; this is useful for patients with renal impairment or at extremes of weight. LMWH effects cannot be reversed with protamine. Side-effects of heparin can include hyperkalaemia and Heparin-induced Thrombocytopaenia (HIT). HIT has a frequency of 2.6% when unfractionated heparin is used, and 0.2% with LMWH (12). The most severe form of HIT is immune-mediated and can be complicated by thrombosis. Platelet counts should therefore be monitored in patients receiving heparin. If HIT occurs, heparin should be discontinued and an alternative anticoagulant used to reduce the risk of thrombosis. Warfarin should not be given to patients with HIT because of the high risk of warfarin necrosis, so alternatives such as lepirudin or danaparoid are chosen. A screening test for antibodies is the initial investigation when HIT is suspected. Patients testing positive proceed to the more specific serotonin release assay. For post-CPB reversal of heparin, protamine sulphate is used. 1 mg of protamine neutralises the effect of 100 i.u of unfractionated heparin, by combining with heparin to form an inactive salt compound. Protamine acts within 5 minutes and can last for 2 hours. Effective protamine reversal is confirmed when the ACT returns to the baseline value. Excess administration of protamine is undesirable, as it has its own intrinsic anticoagulant effect at high doses. It should be given slowly to minimise cardiovascular side-effects (systemic arterial vasodilatation and pulmonary arterial vasoconstriction), and the maximum administration rate in adults is 50mg per 10 minute period. Protamine can only partially reverse the anticoagulant effect of LMWH.

Other indirect thrombin inhibitors
Warfarin reduces hepatic synthesis of FII. Factor Xa inhibitors will reduce activation of FII. These agents are considered by some sources to be indirect thrombin inhibitors and are covered elsewhere in this chapter.

Direct thrombin inhibitors
This class includes Dabigatran, Argatroban, Bivalirudin, the recombinant Hirudins (eg lepirudin and desirudin) and also Melagatran and its prodrug Ximelagatran. Dabigatran is a direct thrombin inhibitor, taken orally, which is being increasingly used in prevention of thromboembolism following lower limb joint replacement surgery. Dabigatran is also gaining a role in prevention of embolic stroke in patients with atrial fibrillation. The predictable pharmacokinetic profile allows a fixed-dose regimen to be used without routine coagulation monitoring. Dabigatran cannot be reversed. It should be discontinued at least 24hrs before elective surgery. In the event of life-threatening haemorrhage, non-specific prohaemostatic agents such as recombinant factor VIIa and prothrombin concentrate complexes could be considered; accelerated clearance using haemofiltration or charcoal filtration has also been suggested (13). Argatroban is given intravenously and has a role in prevention/treatment of thrombosis and during Percutaneous Coronoary Interventions (PCI) in patients with HIT. It is metabolised hepatically so may be more appropriate than Lepirudin for patients with renal impairment. Its effect can be monitored by APTT. Hirudins are direct thrombin (FIIa) inhibitors derived from chemicals found in leech saliva. Lepirudin and Desirudin are recombinant agents. Lepirudin is used to anticoagulate patients with HIT. It is given intravenously, titrated according to APTT (14), and cleared renally. Desirudin can be given subcutaneously to prevent thromboembolism. Bivalirudin is a hirudin analogue, and can be used in combination with antiplatelet agents for patients undergoing percutaneous coronary intervention (PCI). It can be titrated according to ACT (14). Thrombin function recovers to normal approximately 1hr after Bivalirudin infusion is discontinued. Melagatran and its prodrug Ximelagatran were withdrawn from the market after the EXTEND trial found potential risk of severe liver injury (15).

Factor Xa inhibitors
The synthetic pentasaccharide Fondaparinux is an indirect FXa inhibitor, with structural similarities to heparin. It works by facilitating antithrombin III (AT) activity, and is therefore dependent on AT for its effect (16). Fondaparinux is given subcutaneously for prevention/treatment of venous thromboembolism, and for acute coronary syndrome. In patients with normal renal function, anticoagulation effects persist for 2-4 days after discontinuation of Fondaparinux. There is no antidote available to reverse the effect of Fondaparinux. It does not affect PT, APTT or ACT. Although monitoring is not routinely required in healthy patients, it can be achieved by an anti-FXa assay. Newer orally-administered drugs Rivaroxiban and Apixaban are direct FXa inhibitors. They have similar indications to Fondaparinux.

Heparinoids
Danaparoid is produced from the same porcine material as heparin, but all heparin and heparin fragments are extracted. It catalyses AT-mediated inhibition of FXa and, to a lesser extent, FIIa (17). It has been used to anticoagulate patients with HIT. There can be crossreactivity so patients should continue to have platelet count monitored (18). Coagulation monitoring, if required, is by anti-Xa assay. It is not reversed by protamine.

Warfarin
This oral agent is used to minimise the risk of embolic stroke in patients with valvular heart disease or arrhythmia, and to prevent/treat venous thromboembolism. It inhibits the hepatic synthesis of vitamin K-dependent coagulation factors (FII, FVII, FIX, FX) as well as proteins C, S and Z (19). It takes at least 48hrs to develop the anticoagulant effect, during which time the inhibition of proteins C and S may paradoxically create a prothrombotic state, hence heparin is usually added during warfarin introduction. Warfarin effect is titrated using PT or INR tests with target ranges shown in the table. Additional or alternative agents may be recommended in valvular or structural heart disease (20). Perioperatively, warfarin is only continued for certain procedures with low risk of bleeding. Warfarin is usually held 5 days prior to surgery with INR ≤1.5 confirmed immediately preop. A shorter-acting anticoagulant, such as LMWH, can be given as bridging anticoagulant therapy until shortly before surgery and continued once haemostasis has been obtained. The decreased risk of thrombosis or embolism by using bridging anticoagulation is weighed against the increased risk of haemorrhage. Decisions regarding perioperative anticoagulation are made on an individual patient basis (22). www.intechopen.com

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In the event of major haemorrhage in a patient who is anticoagulated with warfarin, it may be necessary to discontinue warfarin, administer vitamin K 5-10mg (phytomenadione), and in some cases replace the deficient coagulation factors using prothrombin complex concentrate 30-50 units/kg (containing FII, FVII, FIX, FX) or fresh frozen plasma 10-15ml/kg.

Antiplatelet agents
These agents have a variety of mechanisms of action as shown in the table. Decisions regarding perioperative discontinuation of these medications are made on an individual patient basis. As with other anticoagulants, continuation increases perioperative bleeding but withholding the agent increases risk of thrombosis. The risk of withholding antiplatelet agents in the setting of acute coronary syndrome generally outweighs the benefits so they are usually continued in this context (23).

Prostacyclins
Epoprostenol is a prostacyclin that can be infused to treat pulmonary hypertension, or to inhibit platelet aggregation during renal dialysis.

Antifibrinolytic Agents
Aprotinin, tranexamic acid and aminocaproic acid are antifibrinolytic agents, which can be used to decrease peri-operative bleeding. Aprotinin is a naturally-occurring serine protease inhibitor, and the other two agents are lysine analogues. Studies have shown reduction in blood loss and transfusion requirements with the use of these agents, particularly in patients taking antiplatelet agents perioperatively (25,26). Aprotinin was associated with increased incidence of death, cerebrovascular accidents and renal events in an observational study of patients undergoing revascularisation surgery (27). These effects were not seen with tranexamic acid or ε aminocaproic acid. Indeed a prior meta-analysis of aprotinin use in patients undergoing cardiopulmonary bypass suggested it was safe (28). The current recommendation is to reserve aprotinin use for patients at the highest risk of bleeding complications (29).

Platelet count
The normal range is 150-400 x10 9 / L. Thrombocytopenia can occur as a result of decreased bone marrow production or excessive destruction. Typically, traumatic bleeding, purpura and easy bruising occur at platelet counts less than 50 x10 9 /L; spontaneous bleeding may occur at platelet counts below 20x10 9 /L. There is not a strong evidence base for a transfusion trigger in thrombocytopaenia (30), however the consensus is that a platelet count of >50 x10 9 should be achieved prior to invasive procedures (31) with a higher target of >100 x10 9 prior to neurosurgery.

Platelet function
Platelet function can be impaired by low pH, hypothermia, myeloproliferative disease, uraemia, non-steroidal anti-inflammatory drugs and antiplatelet drugs. As well as having a sufficient number of platelets, adequate platelet function is also necessary for haemostasis. Platelet function is not measured in standard laboratory tests, but it is possible to measure platelet activity using methods such as aggregometry. Platelet function can be inferred from

Prothrombin time (PT) and international normalized ratio (INR)
Blood is sampled into a tube containing EDTA or citrate, both of which chelate calcium and prevent the blood clotting en route to the laboratory. The sample is centrifuged in the lab to yield platelet-poor plasma. The test starts when calcium and thromboplastin (consisting of tissue factor and phospholipid) are added to the plasma, and is complete when fibrin strands are formed.

Activated partial thromboplastin time (APTT) and aptt ratio (APTTR)
The blood sample for this test is prepared in the same way as that for the PT. The resultant platelet-poor plasma is triggered to coagulate by mixing it with the APTT reagent and replacing the chelated calcium. The APTT reagent consists of phospholipid (as a substitute for the platelet membrane) and negatively-charged particles that cause contact activation. The time taken for fibrin strands to form is the APTT. The APTT ratio is calculated in a similar manner to the INR, with normal APTTR being 1.0 and higher values representing coagulopathy. APTT is prolonged by deficiencies of the Intrinsic or Common pathways, meaning any procoagulant factor deficiency apart from FVII and FXIII will prolong the Underfilling of the sample tube, or raised haematocrit will increase the proportion of citrate (or EDTA) to plasma

Thrombin clotting time (TT or TCT) and hemoclot®
TT measures the time taken for fibrin to form when thrombin is added to the plasma sample. Prolongation of TT suggests low fibrinogen levels, dysfibrinogenaemia or inhibition of thrombin. Thrombin can be inhibited by unfractionated heparin or direct thrombin inhibitors. Fibrin degradation products inhibit fibrin cross-linking; at high concentrations these can also prolong the TT. Adding Protamine sulphate to the sample will negate the effect of heparin and FDPs.
Adding Toluidine Blue will negate the effect of heparin but not FDPs. Another technique for samples containing heparin is to perform a "Reptilase Test", where a snake venon product 'Reptilase' is used to activate fibrinogen. Unlike thrombin, reptilase is not inhibited by heparin.
Hemoclot® is a sensitive, diluted TT assay. These tests are used to assess the effectiveness of fibrinolytic therapy and may be useful in quantifying the effects of direct thrombin inhibitor drugs.

Ecarin clotting time (ECT)
This is a specific assay of thrombin generation. Ecarin comes from snake venom, and specifically activates prothrombin (FII). ECT can be used to monitor anticoagulation during Cardiopulmonary Bypass but is not widely utilised outside a research setting (13) Fibrinogen Fibrinogen (FI), the precursor of fibrin, is synthesised by hepatocytes and normal plasma concentration is 1.5-4.0g/L. Levels vary seasonally within an individual, and causes of abnormal fibrinogen level or function are shown in the tables below. Fibrinogen can be depleted by haemorrhage and consumed by coagulation during or after cardiac surgery, and is routinely quantified by the Clauss method in this setting. The PT-derived fibrinogen level (PT-Fg) is more convenient but less reliable. When investigating congenital fibrinogen defects, more complex and time-consuming tests are employed. There is evidence that the low preoperative fibrinogen concentrations correlate with higher peri-operative blood loss in coronary artery bypass surgery (32,33). Pre-operative prophylactic fibrinogen infusion may be beneficial, although larger trials are required to determine safety and efficacy of this practice (34). Pre-operative hyperfibrinogenaemia is associated with higher all-cause mortality (35), possibly because raised fibrinogen levels are a marker of inflammation, reflecting the presence of a systemic disorder (eg sepsis, acute coronary syndrome).
In major blood loss treated with packed red blood cells and crystalloid/colloid intravascular volume resuscitation, fibrinogen is the first coagulation factor to become depleted (36). It is therefore a useful marker for severity of haemorrhage (37). Fibrinogen quality is assessed when thromboelastometry is performed in the presence of a platelet inhibitor, a test available commercially as "FIBTEM" (see ROTEM section). A prolonged Thrombin Time will also detect low fibrinogen levels.
Options for replacing fibrinogen are fibrinogen concentrate reconstituted to 20g/L, Fresh Frozen Plasma containing 1.6-2g/L fibrinogen, or cryoprecipitate containing 17g/L. The traditional target of replacement of fibrinogen aimed for a level of 1g/L, but higher targets of 1.5-2g/L may be more appropriate in patients undergoing cardiac surgery (38). It remains to be established whether it is better to administer fibrinogen concentrate prophylactically, or to reserve treatment only for patients who are bleeding. There is also debate regarding the most suitable goal for fibrinogen replacement, which may be fibrinogen quantity (eg Clauss assay) or fibrin quality (eg. FIBTEM).

The Clauss assay
A high concentration of thrombin is added to diluted plasma and the clotting time is measured. A reference curve has been plotted using plasma of known fibrinogen concentrations. The fibrinogen concentration of the test sample is determined by comparing the clotting time with the reference curve. The clotting time can be determined automatically using systems that detect changes in light scattering, or light absorption as fibrin is formed. Artefacts can occur when turbid or lipaemic plasma causes abnormal light scattering, and when free haemoglobin or bile cause abnormal light absorption. Heparin will affect clot formation, making the Clauss assay inaccurate, so every effort should be made to obtain a heparin-free sample (avoid sampling within 4 hours of low-molecular weight heparin administration, caution when sampling from arterial lines flushed with heparin). If a heparin-free sample cannot be obtained (eg during Cardiopulmonary Bypass) then ionexchange resins or heparinase can be used to negate the heparin effect. Fibrin Degradation Products (FDPs) affect fibrinogen activity in vivo, and also affect the time taken to complete the Clauss assay, so this assay naturally reflects the any potential impact of FDPs.

PT-derived fibrinogen assays (PT-Fg)
This method indirectly measures fibrinogen by analysing the change in light scattering or optical density as fibrin is formed in the sample during the PT test. The advantage of this method is that it can give a measurement of fibrinogen any time the PT is tested without additional expense. The test may be inaccurate in samples with high fibrinogen and, of particular relevance to Cardiac Surgery, the test may be less reliable than Clauss in the investigation of bleeding diathesis. FDPs have less effect on the PT-Fg than on the Clauss Occurs in 20% of population, leads to 7-10% higher fibrinogen levels than GG genotype (39)

Inherited Fibrinogen Defects (rare):
Congenital Afibrinogenaemia Grossly decreased fibrinogen synthesis, with low or undetectable plasma fibrinogen. Patients have haemorrhagic diathesis, prolonged clotting times and abnormal platelet function Congenital Hypofibrinogenaemia Mild-to-moderate reduction in circulating fibrinogen levels. Potential for haemorrhagic problems, but patient may be asymptomatic.

Congenital Dysfibrinogenaemia
Immunoassays may detect fibrinogen, but tests that depend on production of fibrin strands will give lower fibrinogen results. Patients may have haemorrhagic tendency, thrombophilia or be asymptomatic

Acquired hypofibrinogenaemia
Decompensated liver disease Hepatocellular failure leads to decreased fibrinogen synthesis. Excessive glycosylation of fibrinogen leads to an acquired dysfibrinogenaemia. Excess Fibrin Degradation Products (FDPs) also disrupt fibrinogen function.

Disseminated Intravascular Coagulation (DIC)
Generalised fibrin formation throughout the microcirculation leads to consumption and depletion of fibrinogen. FDP concentration is also raised.

Haemodilution From massive blood transfusion or fluid administration
Systemic Thrombolytic Therapy Usually leads to gross reduction in fibrinogen levels and high risk of haemorrhage assay; an adequate fibrinogen measurement using PT-Fg may not reflect adequate fibrinogen activity if there is a significant quantity of FDPs in circulation.

Clottable protein
This test is too time-consuming and labour-intensive to be used routinely, but does give very accurate results and can be used for reference assays.

Testing for specific coagulation factor abnormalities
Clinical or laboratory evidence of abnormal coagulation that is not explained by haemorrhage, known pathology, or iatrogenic causes may require investigation of specific factor abnormalities. Uncommonly, individuals may have a factor deficiency or a factor inhibitor (the most common example is lupus anticoagulant). Consultation with a haematologist followed by specialised laboratory tests may be indicated.

Other measurements in coagulopathy
For effective coagulation, supportive management to maintain homeostasis is required. Temperature <34°C, pH <7.1, ionised Calcium <0.9mmol/L and Haematocrit <30% have all been shown to adversely affect haemostasis (40).
In coagulopathy, hypocalcaemia should be corrected because of calcium's key role in the activation of FX, FII, FXIII and Fibrinogen. Both haemodilution and chelation of serum calcium by citrate in units of packed red blood cells can contribute to hypocalcaemia in massive transfusion. A value for ionised Calcium can be rapidly obtained from many blood gas analysis machines, and the reference range is 1.0 -1.3 mmol/L (no correction calculation required). Total Calcium ranges from 2.1 -2.7mmol/L (corrected for hypoalbuminaemia may be necessary). Prevention of heat loss and active warming may be required to raise core temperature, acidotic patients may benefit from buffering, and haematocrit should be maintained with transfusion to optimise coagulation. When transfusing Packed Red Cells it is preferable to use blood that has been stored for the minimum possible duration, ideally less than two weeks. Transfusion of older blood has been associated with greater incidence of complications and higher mortality (41).

Near patient coagulation tests
ACT and TEG are the two most common Near Patient Tests (NPT) used in cardiac surgery. The main value of NPT is the short turn-around time, which gives feedback to the treating physician/surgeon without delay. An ACT reading can be obtained generally within 4 minutes of test initialisation, and a TEG within 10 to 15 minutes. A conventional coagulation screen generally has a turn-around time of 30 minutes, during which time one or more interventions may already have taken place, making the result less valuable and potentially obsolete.

ACT (Activated Clotting Time)
This is generally performed at the bedside or in the operating room by a perfusionist or doctor who is actively managing the patient. A pre-heparin baseline value is obtained to assist interpretation of subsequent results. Anticoagulation whilst on cardiopulmonary bypass (CPB) is usually guided by ACT, and the test is also used to assess success of protamine reversal of heparin when CPB is over. After Protamine, an ACT that is prolonged beyond the baseline value suggests heparin reversal may be incomplete, but it offers no information regarding other potential causes of coagulopathy.

Thromboelastography
Thromboelastography was first described in 1948. Viscoelastic changes during coagulation are plotted against time (42). The abbreviation 'TEG' was used for this technique. TEG ® is now a trademark for one manufacturer of thromboelastograph device, an alternative manufacturer uses the trademark ROTEM ® . The principles are similar, described below.

The principles of the thromboelastography (TEG)
Samples for analysis are stored in a citrated tube and must be analysed within 4 hours. Kaolin (a clotting factor) is added to two separate cuvettes both containing sufficient calcium chloride to reverse the effect of the citrate. One of the cuvettes also contains a surface coating of heparinase to deactivate any heparin in the sample. Both cuvettes are loaded onto the analyser for simultaneous analysis. Each cuvette sits on a platform that oscillates through an angle of 4 0 45´. The whole cycle lasts ten seconds including a 1 second rest period. A pin is freely suspended within the blood and is attached to a torsion wire which in turn is attached to a mechanical-electrical transducer. As the blood starts to clot within the cuvette, fibrin strands form between the blood and the pin and the torque is transmitted via the transducer to give a signal that is recorded by a computer (See figure 8).
The variables that are recorded are as follows: "r" (reaction time) is the period of time, in minutes, taken to form the initial fibrin strands after placement of blood in the TEG analyser. Before this happens, the liquid blood in the cuvette exerts very little torque on the pin and the signal remains effectively zero (see figure 8). It is prolonged by congenital or acquired deficiencies in hepatic coagulation factors and anticoagulants such as unfractionated heparin, the low molecular weight heparins and warfarin. The "K" (K time) is the time, in minutes, after clot formation begins and the clot itself is of sufficient strength to produce a 20mm divergence of the line. This reflects the speed of clot strengthening. The "K" is reduced by an elevated fibrinogen and by effective platelet function and is prolonged by the same factors that prolong the "r". The "α" (alpha angle), measured in degrees, is the slope of the TEG tracing with respect to the r line (the line drawn between r 1 and r 2 ). It reflects the speed at which the clot is formed and behaves very much like the "K". If the "K" is undefined (i.e. the signal generated by the computer never reaches an amplitude of 20mm) such as in hypocoaguable states then the "α" becomes more important.

Platelet mapping with TEG
Platelet mapping studies measure the degree to which a patient's platelets can be activated by stimulation of either the thromboxane A2 (Txa2) receptor or the adenosine diphosphate (ADP) receptor. These receptors can be inhibited by aspirin and clopidogrel respectively, but the degree of inhibition varies between individuals. Thrombin activates platelets independent of the arachidonic acid and ADP receptors via the glycoprotein IIb/IIIa receptor. In cardiac surgery, antiplatelet agents are often withheld to minimise bleeding complications, which can expose patients to increased risks of peri-operative myocardial ischaemia. Platelet mapping may have a role in determining the optimum time to operate, such that antiplatelet agents are withheld for long enough to decrease the bleeding risk, but no longer than necessary. In the medical management of coronary artery disease, this technique could potentially be used before antiplatelet therapy is commenced, to determine which antiplatelet agent is likely to be effective for an individual patient. It might also be used after antiplatelet therapy has been initiated to measure the response to therapy. Platelet mapping can be assessed by the following method: 4 TEG channels (cups) are required. Cup 1 contains blood and kaolin (this activates the clotting cascade ex vivo) and the MA reflects thrombin activated platelets (MA thrombin ). Thrombin in the blood sample is produced by trauma during the blood sampling procedure. Since the direct effect of thrombin on the IIb/IIIa receptor is not dependent on stimulation of either the Txa2 or ADP receptors, MA thrombin is an indirect measure of platelet function even in the presence of aspirin or clopidogrel. In all other 3 cups heparin is added to inhibit thrombin thus eliminating its direct action on the IIb/IIIa receptor. To the second cup 10μL reptilase and activated factor XIII (Activator F) is added which convert fibrinogen to fibrin by generating a cross-linked fibrin clot thus isolating the contribution of fibrin to the clot strength.
Activator F has no effect on platelets so the "MA" reflects the action of fibrin only (MA fibrin ) (44). To the third cup 10μL Activator F and 10μL arachidonic acid (AA) are added and the MA AA reflects platelet responsiveness to AA. To the fourth cup 10μL Activator F and 10μL ADP are added and the MA ADP reflects platelet responsiveness to ADP. The relative response of platelets to either AA or ADP is calculated as follows with a normal reference value for each being above 80%: Relative response of platelets to AA = (MA AA -MA fibrin ) / (MA thrombin -MA fibrin ) × 100.
The platelet inhibition in response to an agonist is calculated by subtracting the percentage aggregation from 100 (45).

Rotational Thromboelastometry (ROTEM ® )
ROTEM is based on the same principles as thromboelastography. Citrated whole blood is mixed with reagents in a stationary cuvette, then a pin is placed vertically into the blood and alternating clockwise and anticlockwise forces are applied to the pin. As the blood clots the pin's oscillation is impeded, and when fibrinolysis occurs the pin will move more freely.
The device records the changes in oscillation against time, creating the 'thromboelastogram' (TEM). The zero line indicates free movement of the pin. An amplitude of 100mm would indicate no movement of the pin, meaning the clot has reached the maximum possible firmness measurable by ROTEM. Combinations of reagents can be used to produce a series of TEMs that measure different aspects of the coagulation and fibrinolysis processes.
The ROTEM machine has four channels, thus four TEMs can be generated simultaneously. By using combinations of coagulation triggers and coagulation inhibitors, a clearer understanding of the coagulation abnormalities can be obtained. The reference ranges for the parameters are not interchangeable between the different types of TEM, for example the MCF seen in the FIBTEM test will be significantly lower than that in INTEM or EXTEM, because clot firmness is due to the effect of fibrin only.
Parameters measured using the ROTEM are as follows:

Clotting time (CT)
[Delay from the start of the reaction to the start of clot formation. An amplitude of 2mm is taken to signify the start of clot formation.] This represents: -the initiation of clotting -thrombin formation -the start of clot polymerisation Although not linearly related, CT is related to the PT for EXTEM, and the APTT for INTEM. Abnormalities that prolong the laboratory tests will prolong the CT in the comparable ROTEM test, but the endpoint for the laboratory test is fibrin formation, whereas in ROTEM a degree of fibrin stabilisation is required to attain 2mm amplitude.
Prolonged CT suggests anticoagulant drug activity or coagulation factor abnormalities. Consider: -Anticoagulant inhibitors such as protamine.
-Replacement of coagulation factors (eg. specific factor concentrates, or fresh frozen plasma)

Clot Formation Time (CFT)
[Delay from start of the reaction (2mm) to formation of a 20mm clot.] CFT represents fibrin polymerisation, and stabilisation of the clot by activated platelets and FXIII. Prolonged CFT signifies a hypocoagulable state, and a short CFT corresponds with a hypercoagulable state.
Prolonged CFT may represent: -deficiency of platelet quantity or function -low fibrinogen level -deficiency of fibrin polymerisation Consider: -fibrinogen replacement (fibrinogen concentrate, cryoprecipitate, fresh frozen plasma) -platelet transfusion

Alpha(α)-angle
[Angle formed between the x axis and a line that forms a tangent with the temogram curve and crosses the x axis at the point where clotting starts (2mm)] This describes the kinetics of clotting.
A reduced -angle will correspond with a prolonged CFT. Increased angle will be seen in a hypercoagulable state.
Reduced angle indicates a hypocoagulable state, consider the same causes and management as described for CFT.  If abnormalities of EXTEM are corrected on APTEM they are attributable to hyperfibrinolysis and may be correctable with antifibrinolytic drugs.

A (x) values
A suitable combination of TEMs for a patient undergoing cardiac surgery would be INTEM, FIBTEM and HEPTEM. The INTEM is performed to look at CT (prolonged in consumptive coagulopathy and the heparin effect on the intrinsic pathway) and also the α-angle and MCF (to assess whether fibrinogen or platelet supplementation is needed). The HEPTEM is compared with INTEM to determine the degree of Heparin effect. FIBTEM is performed to define the fibrinogen contribution to the haemostatic plug. As recommended at the European ROTEM meeting 2007, a minimum of 3 sets of ROTEM should be performed; a baseline, a set whilst on CPB, and one after CPB. Further sets are to be performed if the patient is still bleeding. These tests can be performed quickly and interpreted safely between 10-15 minutes from the start of the tests. The FIBTEM test is used clinically to guide administration of fibrinogen (46), but evidence for the treatment trigger or therapeutic target has not been conclusively established. It has been suggested that a FIBTEM MCF of 7mm is adequate for haemostasis in orthopaedic surgery, but a higher target of 22mm has been discussed for cardiac surgery. Caveats for ROTEM use: The INTEM and EXTEM are not very sensitive to mild coagulation factor deficiencies, and are insensitive to defects of primary haemostasis (platelet aggregation). The EXTEM test may still be normal when INR is as high as 4, and may show pathological values with very high heparin levels.

Thromboelastography (TEG) versus Rotational Thromboelastometry (ROTEM)
As discussed above, the techniques have a common origin. Both systems generate graphs of clot firmness between a cuvette and pin versus time. In the TEG system the cuvette oscillates around a pin throughout the clotting process, whilst in the ROTEM system the cuvette is fixed and the pin rotates. By convention, the parameters obtained from the graphs have been given different names in the two tests. A comparison of the nomenclature is given in the table below from an article by Luddington (42).

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Clinical application of near patient test results and evidence base for decision-making
There are several studies in the literature that highlight the usefulness of the TEG in cardiac surgery. Shore-Lesserson et al demonstrated in a prospective randomized controlled trial of high risk cardiac surgical patients (n=105) that fewer patients randomized to the TEG arm were transfused fresh frozen plasma (p < 0.002) or platelets (p < 0.05) compared to those patients managed by standard coagulation tests in the postoperative period (47). Total blood product transfusion was less in the TEG arm and this reached statistical significance. TEG was used to guide protamine, platelet, FFP and antifibrinolytic therapy. The authors concluded that TEG -based transfusion algorithm reduced transfusion requirements. Spiess et al assessed the usefulness of TEG in predicting postoperative haemorrhage and need for reoperation. They found that TEG was a better predictor (87% accuracy) than coagulation profile (51%) or activated clotting time (30%) (48). Another study by the same author concluded that costs and risks associated with allogenic blood transfusions could be reduced with the introduction of a TEG based haemostasis protocol in cardiac surgery (49). Tuman found that TEG predicted postoperative haemorrhage more accurately than routine coagulation tests (88% versus 33% respectively) (50).
Avidan et al (51) showed in a prospective randomised comparison trial that following algorithms based on point of care tests or on structured clinical practice with standard laboratory tests did not decrease blood loss, but did reduce the transfusion of all blood components (p < 0.05) after routine cardiac surgery, when compared with clinician discretion. Finally, Westbrook et al showed in a prospective randomised controlled study a nonstatistically significant trend toward less blood product usage in a strict protocol utilizing TEG compared to physician's choice based on clinical experience and standard laboratory coagulation tests. The study also demonstrated a cost saving benefit (52). Use of an automated protamine titration system, such as the commercially available pointof-care Hepcon®, has been shown to be associated with higher heparin and lower protamine doses. This can potentially decrease activation of the coagulation and inflammatory cascades, with possible advantages of decreased postoperative bleeding and blood product requirement.(23)

Blood product administration in cardiac surgery and the cardiothoracic ICU
If a patient bleeds in the postoperative period, it is important to distinguish between 'surgical' and 'medical' bleeding. A 'medical bleed' from the microvasculature occurs due to a coagulation abnormality, and this abnormality must be detected and corrected with blood products or pharmacological agents to halt the bleeding. In contrast a 'surgical bleed' is from a cardiac or large vessel injury and will often require resternotomy to re-establish haemostasis surgically. Misdiagnosis of 'surgical bleeding' can lead to unnecessary resternotomy, whilst failure to diagnose a 'surgical bleed' may lead to delay and futile use of blood products. A progressive, untreated 'surgical bleed' will lead to a combined 'medical' AND 'surgical' bleed. Likewise, a 'medical bleed' that is not correctly treated may result in a pericardial tamponade that needs surgical intervention. Factors to be considered in post-operative haemorrhage include: -Complexity of surgery and adequacy of surgical haemostasis at end of operation -Baseline coagulation status (APTT, PT, platelet count, ACT or TEG) compared with post-operative measurements -Anticoagulation therapy pre-operatively (Antiplatelet therapy, Warfarin, Heparins, FXa or thrombin inhibitors) -Intraoperative blood loss and blood product replacement -CPB duration -Intra-operative anticoagulant use and reversal -Pump blood usage (this contains additional heparin, thus may require further protamine reversal). -Core temperature -Blood pH -Ionised calcium level Where possible, administration of blood products should be minimised (53). With donor screening and testing of donated blood, the risk of blood-borne disease transmission has decreased. Uncertainties regarding prion-related disease transmission remain, and there is also evidence that blood transfusion during cardiac procedures is associated with worse short-and long-term outcomes (41,54 Mediastinal shed blood reinfusion, following washing and centrifugation, can be considered (direct reinfusion not recommended)  Therapeutic PEEP trial to decrease post-op bleeding is not well established Choice of fluid for intravascular volume replacement can also affect coagulation. Colloidal plasma substitutes may interfere with haemostasis in non-specific or specific ways. Nonspecific effects relate to haemodilution, with decreased circulating concentrations of coagulation factors, platelets and red blood cells. Specific effects are discussed by colloid type. Albumin inhibits platelet aggregation directly (56), and may affect fibrin polymerisation, but in vitro is considered not to affect haemostasis (57). Dextrans are not widely used due to potential allergic, renal and haemostatic side effects. They cause platelet dysfunction by decreasing vWF, possibly by adsorption. Dextrans also accelerate activation of fibrinogen, facilitate fibrinolysis, and may coat endothelium and platelets thus decreasing platelet adhesion. Gelatins can impair platelet coagulation (58) and decrease vWF, but are generally accepted not to influence perioperative bleeding (57). Hydroxyethyl starches (HES) induce platelet dysfunction through effects on GPIIb-IIIa expression and on vWF and FVIII levels. HES can also favour fibrinolysis. Haemorrhagic complications have been reported more frequently with high molecular weight (Mw) HES than with lower Mw. Use of colloids outside the context of clinical studies has recently been questioned as well (59). When using crystalloid alternatives, a balanced salt solution may offer less disruption of haemostasis than saline, possibly due to maintenance of plasma calcium concentrations.

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Perioperative Considerations in Cardiac Surgery 206

Conclusion
Perioperative management of coagulation needs to be tailored to the needs of the individual patient and delivered within the available resources of the Cardiac Surgery Centre. Appropriate use of medication and blood products can minimise perioperative thrombosis and bleeding, limit exposure to potentially harmful blood products and thus maximise quality of patient care.

Acknowledgement
Supported by St. Vincent's Anaesthesia Foundation.